Metal-oxide-semiconductor field-effect transistors (MOSFETs) can be used to amplify or switch electrical or electronic signals. MOSFETs have almost entirely displaced various other switching and amplification devices due to their low unit cost and ease of fabrication, particularly for digital switching applications. A MOSFET can include a metal or polycrystalline silicon (polysilicon) gate separated from a semiconductor region by an insulator, so the term “MOSFET” can refer generally to a FET including a metallic or a non-metallic gate material. The semiconductor region generally includes a substrate of a first conductivity type, and a source region and drain region of a second different conductivity type located on either side of the semiconductor region, under the insulator.
MOSFET devices can be categorized as n-channel or p-channel devices, and as enhancement-mode or depletion-mode devices. The enhancement-mode MOSFET includes a drain region and a source region isolated by the substrate. In the enhancement MOSFET, as voltage is applied to the gate, a channel forms on the surface of the semiconductor region between the drain and the source, allowing current to flow between the source and the drain.
In contrast, the depletion-mode MOSFET includes a coupled source and drain region extending below the gate. Here, as voltage is applied to the gate, a depletion region forms under the insulator, narrowing the coupled region between the source and the drain, the narrowed region reducing the ability for current to flow between the source and the drain.
The terms “n-channel” and “p-channel” refer to the type of charge carrier providing conduction between the source and drain regions. An “n-channel” or “NMOS” device uses majority conduction via electrons when the device is biased into conduction. Similarly, “p-channel” or “PMOS” refer to conduction via the migration of “holes.” Unlike bipolar junction transistors (BJTs), MOSFETs use majority carriers primarily.
Different types of MOSFET devices can be co-integrated on a single monolithic substrate, such as by fabricating one or more wells of a first conductivity type (e.g., n type) within a substrate of the opposite conductivity type (e.g., p type). Such integrated combinations are called complimentary metal-oxide-semiconductor (CMOS) integrated circuits.
CMOS integrated circuits are usually more cost effective to manufacture as compared to bipolar technology. Also, CMOS integrated circuits can be planar, including processing primarily involving one surface of a substrate or wafer. Such planar processing can include, for example, ion implantation, diffusion, deposition, oxidation, epitaxy, one or more photolithographic techniques, or one or more other process steps. Multiple MOSFETs, among other devices such as MOS capacitors or resistors, can be fabricated and interconnected on a single monolithic substrate. Such integrated assemblies can include anywhere from a handful of devices to beyond hundreds of millions of individual devices.